This invention relates to cardiac diagnostic and chronic therapeutic leads, and more particularly to fixation leads having an anchoring element and capable of delivering drugs near the point of fixation.
Cardiac pacing leads are well known as a means for carrying pulse stimulation signals from pacing devices to the heart, and for monitoring heart electrical activity from outside the body. Typically, such leads are sufficiently flexible and small in diameter to allow intravenous introduction into a cardiac cavity, whereupon an electrode at the distal end of the lead is implanted into the endocardium to secure the lead. For this purpose, helical coils, barbs or other tissue piercing fixation elements often are provided, typically as part of or integral with the electrode.
The anchoring or fixation element must be sufficiently sharp to pierce and penetrate the endocardium and secure the electrode against becoming detached, for example due to contractions of the myocardium. During a critical period immediately after implant and prior to full fibrotic growth, usually three to twelve weeks, the anchoring element must provide substantially the entire force maintaining the electrode in its chosen location. Moreover, the interaction between the electrode and body tissue at the point of implant gives rise to both acute and chronic increases in the stimulation threshold. Consequently, pacing devices must be designed to deliver pulses of sufficient amplitude to exceed the increased threshold, or an attempt is made to minimize the increase.
One technique for reducing stimulation thresholds is the elution of a drug in the area of implant. For example, U.S. Pat. No. 4,577,642 (Stokes) discloses a drug dispensing body implantable lead in which an electrode member 16 includes a recess for housing a molecular sieve material which retains a drug, alternatively a solid plug or a powder. The electrode tip provides a sintered elution path which regulates the dispensing of the drug to counteract the chronic increase in stimulation threshold. Another approach to increasing lead sensitivity and attempting to minimize the increase in stimulation threshold is to provide a porous electrode tip. For example, U.S. Pat. No. 4,408,604 (Hirshorn et al) discloses a pacemaker electrode tip, with apertures 19 through the tip. These apertures are said to improve attachment of the electrode to surrounding tissue, as well as to reduce the sensing impedance of the tip.
These prior art approaches, while suitable for certain applications, fail to address the need for rapid drug delivery to counter acute threshold increase, and for the positive fixation of a lead distal tip in combination with drug elution near the tip to reduce both acute and chronic stimulation threshold.
Therefore, it is an object of the present invention to provide a positive fixation cardiac pacing lead adapted for rapid drug delivery immediately upon implant to counter an acute stimulation threshold increase.
Another object is to provide a positive fixation cardiac pacing lead having a porous electrode for local delivery of a therapeutic drug upon implant, in combination with a drug impregnated matrix for chronic drug delivery.
Another object of the invention is to provide a positive fixation cardiac pacing lead containing a drug impregnated matrix for chronic drug delivery, and having an electrode tip particularly configured for increased sensitivity to cardiac electrical activity.
Yet another object is to provide a positive fixation cardiac pacing lead constructed for convenient, proximal end loading of a drug delivery matrix into the coupling structure between the lead electrode and distal end of a lead conductor.